1. Field of the Invention
This invention relates generally to a method for separating wafer-level packages from a wafer and, more particularly, to a method for separating wafer-level packages that use lateral interconnects from a wafer that includes precutting a cover wafer between the wafer-level packages before the cover wafer is mounted to a substrate wafer on which integrated circuits are provided.
2. Discussion of the Related Art
It is known in the art to provide wafer-level packages for integrated circuits, such as monolithic millimeter-wave integrated circuits (MMIC), formed on a substrate wafer. In one wafer-level package design, a cover wafer is mounted to the substrate wafer using a bonding ring so as to provide a hermetically sealed cavity in which one or more integrated circuits are provided. Typically, many integrated circuits are formed on a substrate wafer and covered by a single cover wafer as a batch integration, where each integrated circuit is surrounded by a separate bonding ring. The cover wafer and substrate are then diced between the bonding rings to separate the packages for each integrated circuit. The dicing process typically uses a wafer saw that cuts the cover wafer between the packages where a portion of the cover wafer may be removed. The substrate wafer is then cut between the packages.
For these types of wafer-level packages, interconnects to the integrated circuit within the cavity are typically made in two ways. In a first technique, vertical vias are provided through the cover wafer for a connection to electrical signal traces within the cavity. In a second technique, a lateral interconnect is provided where signal and ground traces extend through the bonding ring and are accessible laterally from the integrated circuit outside of the package. Lateral interconnects are traditionally very difficult to implement using wafer-scale assembly and bonding methods because contact pads for the circuits are relatively inaccessible. Further, the cover wafer is very thin. Therefore, when dicing the cover wafer between the packages, the saw blade sometimes cuts through the lateral interconnects severing the connection.
Probe pads are generally provided on the lateral interconnect for testing and probing purposes to allow the integrated circuits to be tested at the wafer level. Thus, the probe pad needs to be exposed to allow access thereto. For lateral interconnects, this requires that a portion of the cover wafer between the packages be removed. In an alternate process, deep reactive ion etching (DRIE) or dry etching methods can be employed to etch the substrates so the probe pad is exposed. However, such an etching process is typically complex because etching through the substrates is material dependent, where a different chemistry is required, and the process is typically very slow and costly. Further, proper protection to the etching chemicals or gas may need to be implemented to protect the probe pads and the thin-film layers beneath. Also, depending on the substrate material and thickness, dry etching may be impractical or impossible. For example, quartz substrates cannot be etched easily with a dry etch, and is almost impossible to etch quartz substrates greater than 300 μm with a dry etch.